Many varieties of trunked two-way radio communications systems are known. FIG. 1 is a block diagram illustrating both a typical conventional radio system 101 and a trunked radio system 103.
In the conventional radio system 101, a plurality of mobile radio subscriber units 110, 112, 114, 116, 117, 118, 120, 122, 124, 126 are formed into talkgroups A, B or C. Each talkgroup uses a separate channel for communication. The channels in use in FIG. 1 are shown as ‘Channel 1’, ‘Channel 2’ and ‘Channel 3’. Thus, each talkgroup is served by one channel.
In contrast, the trunked radio system 103 and its mobile radio subscriber units 110, 112, 114, 116, 117, 118, 120, 122, 124, 126 use a pool of channels, ‘Channel 1’, ‘Channel 2’ or ‘Channel 3’. All talkgroups may in fact be served by any channel, and may well be served by all channels at different times. These channels can support a virtually unlimited number of talkgroups. The trunked radio system 103 works to take advantage of the probability that not all talkgroups will need a channel for communication at the same time. Estimates are made about how much load a typical user will present to the system, in terms of calls per hour and duration of each call.
For a given traffic load, the trunked radio system 103 requires fewer channels, since all talkgroups can be served by all channels. The number of trunked channels required to provide satisfactory service depends on: (i) the number of users; (ii) the traffic load that each will present; and (iii) the acceptable quality of service (QoS). With any given number of channels, the trunked radio system 103 can accommodate a much greater number of talkgroups than conventional radio systems, such as radio system 101. Hence, a primary benefit of a trunked radio system is the efficient utilization of channels. The trunked radio system allows more users to carry on more conversations, over fewer distinct channels. This applies to data and/or voice calls.
As seen in FIG. 2, a trunked radio system can be either a ‘centralized trunked’ radio system 201 or a ‘decentralized trunked’ radio system 203.
A centralized trunked radio system 201 uses a dedicated or exclusive channel for communication between mobile radio subscriber units 210, 212, 214, 216, 218, 220, and a central controller 205. This dedicated channel is often referred to as a control channel. The control channel communicates information about call ‘set-up’ and ‘tear-down’ between the mobile radio subscriber units 210, 212, 214, 216, 218, 220, and the central controller 205. Other terms that sometimes refer to the central controller 205 include ‘trunking controller’, ‘site controller’, ‘resource allocator’, ‘channel allocator’, ‘controller’, and other like terms. The mobile radio subscriber units 210, 212, 214, 216, 218, 220 constantly monitor the control channel for channel assignment instructions. In order to start a group call, i.e. a one-to-many call, a mobile radio subscriber unit requests that a channel be allocated for its use. The central controller 205 then transmits instructions, which tell the mobile radio subscriber units in the group to switch to a traffic channel ‘Channel 1’, ‘Channel 2’, ‘Channel 3’ or ‘Channel 4’ assigned for that call. A similar process is followed when a mobile radio subscriber unit starts an individual call, i.e. a ‘ one-to-one’ call.
A decentralized trunked radio system 203, however, does not require the use of an exclusive channel ‘Channel 1’, ‘Channel 2’ or ‘Channel 3’. The intelligence or control function for assignment of a channel to a call remains in the mobile radio subscriber units 210, 212, 214, 216, 218, 220. Thus, the decentralized trunked radio system 203 can co-exist with conventional users on the same channels, without the use of the control channel. When a call is initiated by a mobile radio subscriber unit, the channel assignment is determined by the logic in the mobile radio subscriber units 210, 212, 214, 216, 218, 220, not by a controller. In operation, a mobile radio subscriber unit scans the channels, finds an idle channel and starts a call on the idle channel. The disadvantage of the decentralized trunked radio system 203 is that the scan to find an idle channel significantly increases the access time, which often provides for unacceptably high latency delays during call set up.
In a trunked communication system, each call is assigned one channel. The channel comprises two frequencies. At a base station of the trunked communication system, one frequency is used to receive a call from a mobile radio subscriber unit. This first frequency is referred to as the ‘Rx’ or ‘receive’ frequency. The function of the base station is to re-transmit the call to the other members of a talk group. A second frequency is used for that re-transmission. This second frequency is referred to as the ‘Tx’ or ‘transmit’ frequency.
Henceforth the term ‘mobile radio subscriber unit’ will be used for any wirelessly linked mobile communication device that may be linked to the trunked wireless communication system discussed in the remainder of this description. The mobile radio subscriber unit may, for example, be a mobile or portable radio, or another wirelessly linked mobile communication device. The trunked radio system may comprise an ‘infrastructure’. The infrastructure typically comprises a network of linked base stations. A base station communicates directly with mobile radio subscriber units. The mobile radio subscriber units can communicate with each other, via the base station. The mobile radio subscriber units may also be able to place and receive calls through to other, separate communication systems.
Many currently deployed trunked radio systems use a dedicated control (data) channel, common for all mobile radio subscriber units in a particular site, and multiple voice channels. The voice channels are dynamically assigned by the infrastructure of the system to the different talkgroups. This is the design explained as the ‘centralized trunked system’ in FIG. 2.
In typical applications, the radio frequency (RF) receiver in a mobile radio subscriber unit must tolerate large interfering signals. Some interfering signals emanate from other mobile radio subscriber units that are actively communicating in adjacent channels. Another source of interfering signals may be transmissions from sources with large transmission power, even if those transmissions are relatively far removed in frequency. Here ‘relatively far’ means that the transmission frequencies are further from the mobile radio subscriber unit's reception frequency than just the immediately adjacent channels. Such high powered transmissions, even in very different frequencies, can cause significant interference problems.
The interfering signals lead to one or more large undesired signals being introduced into a mobile radio subscriber unit's receiver RF passband. When this happens, the receiver gain drops. In addition, the receiver's noise level increases, due to the resulting compression and nonlinearities. The consequence is that there is degradation of the signal-to-noise ratio and the receiver sensitivity. This phenomenon caused by the large close proximity interference signals is called ‘blocking’. One case when blocking affects the user experience may, for example, occur when two or three radios transmit simultaneously, in close proximity. These may be radios that belong to different talk groups of the trunked radio communication system. A receiving mobile radio subscriber unit that is also in a close proximity cannot then access the radio trunked system, due to its receiver being blocked.
A known approach to reducing interference in links between a base station and mobile radio subscriber units is to evaluate parameters relating to the quality of individual communication links. After such evaluation, transmission attributes can be selected so as to optimize transmission quality of each link.
Accordingly, there is a need for a method and apparatus for transmission in a trunked radio communication system, whereby some or all of the above disadvantages are overcome.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.